The present invention relates to a charge pump circuit for generating a negative voltage. The present invention also relates to a circuit for generating a negative voltage that can be provided to a substrate gate of a MOS transistor in a monolithic IC.
Recently there are many devices in, for example, cellular phones, that need a negative voltage. For example, there is a device in which a substrate gate (back gate) of a MOS transistor receives a negative voltage to restrict a leak current that would otherwise be generated by miniaturization. For example, where a Li-ion battery is used as a positive voltage power source, a circuit is needed to generate a negative voltage from the positive voltage.
FIG. 4 is a circuit diagram illustrating a conventional reversal charge pump circuit for generating a negative voltage. In FIG. 4, a charge pump circuit 100 includes a control circuit 105, switches SW101, SW102, SW103, SW104, configured as MOS transistors, and driver circuits 101, 102, 103, 104 that correspond to the switches. The control circuit 105 operates the switches SW101-SW 104 through the driver circuits 101-104.
At first, the control circuit 105 turns on the switches SW101, SW102 and turns off the switches SW103, SW104. At the time, a capacitor C101 is charged with a voltage value that is Vin−Vc. Subsequently, the control circuit 105 turns off the switches SW101, SW102 and turns on the switches SW103, SW104. Thus, a capacitor C102 is charged with a negative voltage that has the voltage value of the capacitor C101, but reversed in polarity. As a result, the negative voltage is provided as an output voltage Vout. The output voltage Vout becomes −(Vin−Vc) when there is no load.
A voltage value of a connection CN becomes a negative voltage −(Vin−Vc). A voltage value between the gates of the switches SW102, SW104 and the connection CN becomes maximum 2×(Vin−Vc), when a power source voltage of the switches SW 102, SW 104 is the input voltage Vin. As a result, the voltage value may exceed a breakdown voltage of the MOS transistors. To prevent the voltage value from exceeding the breakdown voltage of the MOS transistors, it may be necessary to provide a complex voltage control or an output voltage of a constant voltage circuit such as the input voltage Vin. Thus, there are problems with the prior art which may require an increase in man-hours of circuit design or an increase in the size of the circuit.
Furthermore, a voltage range of the input voltage Vin includes 3.2 V−4.4 V, when the input power source uses a Li-ion battery. The voltage of the connection CN includes −4.4V-−3.2V, when a common voltage is 0V. As a result, the output voltage Vout has a large ripple and becomes unstable. Thus, a different voltage between the positive voltage and the negative voltage of the charge pump circuit 100 becomes twice the input voltage Vin and more than the breakdown voltage of the MOS transistors, when the common voltage Vc includes 0V. Therefore, the size of a chip may need to be increased, when the breakdown voltage is raised by a manufacturing process. It may be difficult and time consuming to design the circuit when a voltage control circuit is required to avoid exceeding the breakdown voltage. In addition, the constant voltage circuit needs to change design by a current that is provided from an output terminal, when the charge pump circuit receives the input voltage Vin from the constant voltage circuit. As a result, the size of the chip and the man-hours of circuit design increase.
Japanese Patent Laid-Open No. 2003-259624 shows a power source providing circuit to prevent a problem when the power source providing circuit provides a positive voltage and a negative voltage to a load formed of GaAs FET or MMIC. The prior art power source providing circuit is different than the present invention.